Process for the electrolytic production of hyperpure zinc



Aug. 10, 1965 I sc cc A'n 3,200,055

PROCESS FOR THE ELECTROLYTIC PRODUCTION OF HYPERPURE ZINC Filed June 30, 1961 2 Sheets-Sheet 1 2 1 \Figl INVENTOR.

Filed June 30, 1961 PROCESS FOR THE ELECTROLYTIC PRODUCTION OF HYPERPURE ZINC 2 Sheets-Sheet 2 INVENTOR.

United States Patent 3,2ti0,055 PROCESS FOR THE ELECTROLYTIC PRODUC- TION 0F HYPERPURE ZINC Giovanni Scacciati, Venice, Italy, assignor to Montevecchio Societa Italiana del Piombo e dello Zinco, Milan, Italy, a corporation of Italy Filed June 30, 1961, Ser. No. 121,246 Claims priority, application Italy, July 5, 1960,

1,872/ 60 3 Claims. (Ci. 204-114) It is known that the production of stable zinc alloys, resistant to aging, requires the use of very pure zinc, containing less than p.p.m. (parts per million by weight) of lead. A known process for the preparation of zinc suitable for such purposes involves distillation of the metal. I

Numerous attempts .have also been made to develop electrolytic processes including recourse to the metallurgy of amalgams, by utilizing normal sulfuric baths, or with soluble zinc anodes, i.e., by an electrolytic refining process, or with insoluble anodes, generally consisting of a lead-silver alloy, i.e., by a process of electrolytic extraction, wherein the electrolyte is a manganese-containing zinc sulfate acid solution with an aluminum cathode.

The introduction of diaphragms of various kinds into electrolysis cells had made it possible to obtain some improvements, but raised other difiiculties, whence the results obtained in practice could not be considered as conclusive and satisfactory. Moreover, the diaphragm was always intended only as a simple filtering means in order to prevent the electrolyte and therefore the cathodic zinc deposit from becoming contaminated by the slime formed on the anode and becoming detached therefrom. In other words, the diaphragm had the task heretofore of mechanically preventing the passage of solid particles from the anodic zone to the cathodic zone while permitting passage of electrolyte and'current which is discharged onto the metal surface of the anode.

Under these conditions the lead present in the solution as lead sulfate (PbSO cannot be retained by the diaphragm and deposits together with the zinc.

My invention has as an object the overcoming of these difficulties and the introduction of a completely different principle, namely the utilization of the diaphragm as a subsidiary anode on which part of the current is discharged; at the same time preventing transference of solution from the anodic to the cathodic zone, and thereby creating in the first zone conditions that renderthe solubility of PbSO to practically nil.

FIG. 1 is a vertical longitudinal section of an electrolytic cell;

FIGS. 2 and 3 are vertical cross sections of a cathode and anode respectively;

FIG. 4 is a view of a preferred anode.

Under the above conditions, the lead anode is essentially limited to a current conductor while all the anodic phenomena are transferred to the subsidiary anode constituted by the unit: anolyte present between anode and diaphragm and surface of the diaphragm. More particularly, a manganese dioxide layer is caused to form on the fabric of the diaphragm. This manganese dioxide layer which appears to be electrically conductive, is charged positively like the lead anode and, therefore, acts not only as a diaphragm but also as a subsidiary anode.

An essential contribution to the completion of this result is the concomitant fact that the manganese dioxide deposited renders the diaphragm almost completely impermeable, whence the anolyte enclosed in said diaphragm cannot mix with the remaining electrolyte, causing the anolyte acidity to rise to values much higher than normal,

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reaching concentrations of free sulfuric acid of the order of 250-280 g./l. As a result, the true anode is no longer constituted by the lead sheet as in normal electrolysis cells, but by the unit: lead sheet acting as a main conductor; highly acidic anolyte having a composition different from that of the remaining electrolyte which due: to its high acidity presents a very high electric conductivity; manganese-dioxide coated diaphragm which is also electrically conductive and on which all anodic phenomena (oxygen development, formation of peroxides, etc.) generally displayed by lead take place.

The practical consequence is that, by operating ac cording to my invention, the anodic surface present in the electrolytic process proper for zinc production, is now essentially a layer of manganese dioxide instead of a layer of lead dioxide as normally formed on a lead sheet. Moreover, due to the high acidity existing inside the small bag, there are obtained at the same time the ideal conditions for reducing the formation of PbO to a substantial degree (it has been found in fact that the formation of anode scales is reduced to a minimum) and the solubility of the Pb++ ions in the electrolyte of the cell is lowered to substantially zero.

These conditions are obtained by providing 21 diaphragm having the following characteristics:

(l) The fabric should be capable of resisting a comparatively high acidity (up to 30% of H 50 and should have a very compact weft so as to be difiicultly permeable by the solution at the beginning of the electrolytic process. The MnO layer which, as will be specified hereinafter, forms on the fabric, will then render the fabric practically impermeable to the electrolyte. A fabric having the above characteristics and having proved to be the most suitable in our case is that constituted by very compact vinyl resin monofilament yarns (e.g., Vinyon or Movil), with a weight not lower than 400 g./m

(2) A very high acidity, from 250 to 280 g./l. of H should be attained in the solution contained in the diaphragm. It is known that the solubility of PbSO in the electrolyte for zinc rapidly decreases by increasing the acidity; and becomes practically nil at the afore-mentioned acidity values. Under these conditions, lead, dissolved by anodic attack in the form of PbSO precipitates immediately within the diaphragm and has no possibility of entering the cathodic zone. Obviously, lead separated from the anode in any insoluble form, e.g. PbO or metallic particles mechanically separated, remains within the diaphragm together with the PbSO, precipitate. The above-indicated conditions are realized by placing the diaphragm fabric as close as possible to the anode metal.

This is done by having the diaphragm in the shape of a bag a little larger than the anode, into which bag the anode itself is introduced. The limited volume of the anolyte allows the formation of a high acidity in said anolyte.

(3) Due to the almost direct contact between the diaphragm fabric, of the type and quality described under 1, and the surface of the metallic anode, the diaphragm is covered with an adherent and rather compact Mn0 layer within a comparatively short time, about 2 or 3 days of operation. It is known that the normal electrolyte for zinc contains in solution 1 to 5 g./l. of Mn++.

Manganese dioxide conducts electric current and, therefore, the current passing from the cathode to the anode in the cell is to a great extent discharged on the MnO surface formed.

A composite anode is thus obtained which in its innermost and more protected part is constituted by a normal lead-silver alloy, while its outer part is constituted by an electrically and chemically inert fabric and by an MnO layer on which most of the current is discharged, with a resulting lively oxygen development. This layer obviously cannot yield any lead since it is lead-free. Between the two afore-mentioned surfaces the composite anode presents an intermediate anolyte layer of the high acidity, in which all lead compounds present, if any, are substantiallyinsoluble; t

7 According to my invention the basic conditions for avoiding the transfer of lead in any form from the anode to the cathode and cathodic zinc deposits are obtained whereby the zinc deposited has a lead content less than ppm. and even less than 2-3 p.p.m., i.e., hyperpure Zinc with a purity higher than 99.999%.

The cell according to the present invention will, conveniently operate with comparatively low current density of from 200 to 250 amp/m and the temperature of the invention is represented, the same numerals in the various figures represent the same features. A'indicates the feed of neutral electrolyte, B the discharge of acid electrolyte, 1 the anode, 2 the bag diaphragm, 3 the cathode, 4 the entire cell, 5 are U-shaped members hooked on the traverse 6 to hold the bag 2 in place.

It is understood that the formation of manganese dioxide is made possible by the presence of manganese ions in the electrolyte used. The purity attainable also depends on the purity of the neutral electrolyte fed, which should have a Cd content less than 0.0002 g. per liter and should be Pb-free. The cells may be of conventional material but coated with lead-free antiacidmaterial, such as polyvinyl chloride.

Owing mainly to the greater density of the anolyte as compared with the catholyte, the diaphragms tend to swell thereby losing the required adherence to the lead anode and even contacting the cathode. That inconvenience of PVC-clad iron, holding the bags in place as shown in FIG. 4. The electrolyte should preferably be cooled to -35 C. by conventional means (not shown). The lead sheets are of conventional lead, or lead alloy (e.g. with 1% Ag content).

- The following example is given to illustrate, but not to limit, the invention. 7

In apparatus as described above using as the anode a lead-silver alloy containing' 1% silver which was sur rounded by a compact vinyl resin monofilament yarn, with a weight of 400 g./rn. and using a zinc cathode, the electrolysis of an aqueous zinc sulfate solution was carried out. The electrolyte had a cadmium content of less than 0.0002 gram per liter, a manganous content of 4 g./l. and was free of lead. After an interval of two days, a. manganese dioxide MnO coating was formed on the fiber layer. An electric current which varied from 200 to 250 amp/m. was used. The temperature of the cell was kept below C. The anolyte, that is the eleccan be prevented by providing U-shaped members, e.g.,

. 4 trolyte within the fabric diaphragm had an acidity which varied between 250 to 280 g./l. of H The zinc produced had a purity of 99.999%.

There are many other modifications and variations of this invention in view of the above disclosure. It is therefore to be understooch within the scope of the appended claims, that the invention may be practised otherwise than as specifically described.

I claim:

1. A process for preparing hyperpure zinc containing less than 5 ppm. of lead, by electrolysis of a zinc sulfate solution containing manganous ions, which comprises using an anode consisting of an insoluble lead-containing sheet, said lead sheet being surrounded by a compact diaphragm coated with manganese dioxide'and' having between said lead sheet and said diaphragm an anolyte containing about 250-280g/l. free H SO said manganese coating being formed by said electrolysis;

2. An electrolysis cell for producing zinc, having a lead content less than 5 ppm. of lead, said cell having an anode comprising a lead silver alloy containing about 1% silver, said alloy acting as the main conductor, an anolyte having acidity of 250-280 g./l. of H 80 and a compact fabric diaphragm surrounding said alloy and said anolyte and separating said alloy and said anolyte from the reniaining electrolyte, said diaphragm'being made of synthetic fiber with a texture such that when used in an electrolytic process for producing zinc, anodic phenomena of the electrolytic process take place on said fabric diaphragm rather than on said lead anode surface, and said diaphragm having a conductive layer of manganese dioxide on the surface thereof. e

' 3. An electrolysis cell for producing zinc, having a lead content less than 5 ppm. of lead, said cell having an anode comprising a lead silver alloy containing about 1% silver, said alloy acting as the main conductor, an anolyte having acidity of 250-280 g./l. of H 80 and a compact fabric diaphragm surrounding said alloy and saidanolyte and separating said alloy and said anolyte from the electrolyte solution, said diaphragm being made of synthetic fiber with a texture such that when used in an electrolytic process for producing zinc, anodic phenomena of the electrolytic process take place on said fabric diaphragm rather than on said lead anode surface, said diaphragm having a conductive'layer of manganese dioxide produced by oxidation of manganous ions present in the electrolyte, said diaphragm being held by mechanical means to said lead alloy.

References Cited by the Examiner V UNITED STATES PATENTS 757,817 4/04 Laszczynski 204282 1,299,519 4/19 Stuart 204-119 2,633,452 3/53 Hogaboom et al. 204-295 2,848,399 8/58 McGraw et al. 204--1l9 2,863,810 12/58 Henderson et al. 204-419 2,867,570 1/59 Dufour et al. 204290 WINSTON A. DOUGLAS, Primary Examiner. 7 JOHN R. SPECK, MURRAY TILLMAN, Examiners. 

1. A PROCESS FOR PREPARING HYPERPURE ZINC CONTAINING LESS THAN 5 P.P.M. OF LEAD, BY ELECTROLYSIS OF A ZINC SULFATE SOLUTION CONTAINING MANGANOUS IONS, WHICH COMPRISES USING AN ANODE CONSISTING OF AN INSOLUBLE LEAD-CONTAINING SHEET, SAID LEAD SHEET BEING SURROUNDED BY A COMPACT DIAPHRAGM COATED WITH MANGANESE DIOXIDE AND HAVING BETWEEN SAID LEAD SHEET AND SAID DIAPHRAGM AN ANOLYTE CONTAINING ABOUT 250-280 G./L. FREE H2SO4, SAID MANGANESE COATING BEING FORMED BY SAID ELECTROLYSIS. 